Process of applying thin coats of gelatin containing silver halide to a film



5 A L l m m w Z w R M P d e R n July 30. 1968 PROCESS OF APPLYING THINCOATS OF GELATIN CONTAINING SILVER HALIDE TO A FILM FIG. I

FIG.3

FIG-Z FIG-4 INVENTOR RUDOLPH PRZYBILLA By 20% 8M ATTORNEY Unite 2Claims. cl. 117 34 This invention pertains to a process and apparatusfor applying film-forming solutions and dispersions, especiallyphotographic emulsions, to carrier webs, of any type suited to thepurpose, by the so-called dip-coating or skim-coating process.

It is known in the art that application of filmforming solutions anddispersions to moving webs can be effected by processes in utmostvariety. A familiar and widely used process is the so-calledskim-coating process. In this process the web to be coated is passedaround a roll which is so disposed in a coating pan that the web to becoated comes into contact with the coating solution and picks up orentrains a layer of the film-forming liquid. The thickness of theentrained layer then depends on the speed at which the web to be coatedis conveyed past the coating pan, as well as on the viscosity and thesolids content of the coating liquid; i.e., solution, emulsion ordispersion.

For a flawless coating it is known to be very important that in everycase the layer thickness should remain uniform throughout the length andwidth of the web and not fluctuate or vary in thickness. Even thesmallest variations in layer thickness of a single coating of an aqueousgelatin silver halide emulsion, for example, lead to serious defectswhich, after processing the material, are manifested in the form ofdeviations in darkening, known in the literature of the art as haze. Forexample, when a photographic material in which variations in thicknessoccurred during coating is exposed and developed, deviations in densityare distinctly apparent, and are more conspicuous the oftener variationsoccur between thicker and thinner spots or areas.

These irregularities have been found to have been caused in part by thefact that the coating roll, which in fact is usually at a lowertemperature than that of the coating liquid, exerts a cooling effectthrough the web, e.g., film base on the interface of the coating liquid.Since the thickness of the film base always varies somewhat, the coolingeffect will also undergo local variations. For this reason, viscosityvariations occur in the coating solution or dispersion, leading touneven application thereof. Prior efforts have been made to prevent theundesirable heat transfer between the coating roll and the coatingliquid by heating the coating roll to the temperature of the latter. Butthis heating of the roll entails a substantial additional expense, andalso embodies several new sources of trouble.

Now it has been found in accordance with this invention that theabove-mentioned difficulties can be overcome and excellent results canbe obtained if the skimcoating process is carried out in such a mannerthat during the application of the film-forming solution, emulsion ordispersion, and, if desired, also during transport of the coated webthrough the setting and drying zone of the coating apparatus, there isprovided at least in part a gaseous layer of low thermal conductivity,preferably an air layer, between the coating roll and the web andbetween the coated web and guide or rolls. This gaseous insulatinglayer, at least to a large extent, prevents heat transfer, and thedeleterious effect already mentioned, between the coating roll and thecoating liquid. By this States Patent "ice means the process of thisinvention permits application of the film-forming coating liquid in away that is characterized by improved uniformity of the coated layerthroughout the length and width of the web.

In particular, by the process of this invention, aqueous dispersions ofphotosensitive silver halides in a natural or synthetic water-permeableorganic colloid can be applied to moving webs of film or paper toproduce colloidsilver halide layers of enhanced uniformity.

The process of the invention is equally applicable, however, to coatingwebs with other film-forming solutions used in making photographic filmsand papers, e.g., layers which are not photosensitive, such asprotective coatings, adhesive layers, filter layers and antihalationlayers.

If application by dipping is combined in a manner shown in the art withair jet or air-knife doctoring devices, it is possible by this processto apply coating liquids, i.e., solutions, dispersions or emulsions,even with high solids content, in thin layers and with improved uniformsurface characteristics.

The coating apparatus of this invention that is useful for carrying outthe process has (1) A coating pan, and

(2) A coating roll for transporting a web into surface contact with afilm-forming liquid in the pan; said coating roll being characterized byhaving a plurality of uniformly spaced surface areas of small magnitudeand uniformly depressed areas of small magnitude adjacent the surfaceareas, whereby upon rotation of the roll, a cushion layer of gas, e.g.,air, is provided between the surface of the roll and the web as the webcontacts the liquid coating solution.

Thus, the coating roll can advantageously be made so that its surface issuitably shaped to provide alternate surface areas and depressed areasof small magnitude. In a particularly preferred embodiment, the surfaceof the web may have essentially parallel ribs and grooves, the ribsbeing from about 0.05 mm. to about 0.5 mm. in width and the parallelgrooves being from about 1 mm. to about 8 mm. in width. Usually thedepressions will have a depth equal to the average width between theprojections. The width of the groove can vary in accordance with thenature of the particular web to be coated. For the stiffer webs, thegrooves can be wider. The web should not dip into the grooves and, ingeneral the grooves should not exceed about 8 mm. in width.

Instead of grooves, the surface of the coating roller may be reticulatedhaving a mesh pattern or have knobs or similar raised portionssurrounded by depressed portions.

Again, in carrying out the process according to this invention, thoserolls are suitable in which the jacket of the coating roller hasorifices uniformly distributed over the surface through which air canflow, so that an insulating air cushion is formed between the roll andthe web to be coated. In this case, air can be supplied through a ductin a roll trunnion into the hollow roll and thence through the spacedorifices.

The desired insulating effect of the rolls made in accordance with thisinvention can be still further improved if rolls are employed which aremade entirely, or only the jacket is made, of an organic plasticmaterial of low thermal conductivity, for example, polyvinyl chloride,polymethyl methacrylate, polyethylene or polytetrafluorethylene, or arecoated with such a plastic material. The invention will be furtherillustrated by reference to the accompanying drawings which form part ofthis application.

In the drawings:

FIG. 1 is an elevation of a suitable coating roll having parallelgrooves and ribs;

FIG. 2 is an elevation of a suitable coating roll having knobs;

FIG. 3 is an elevation with structure in dotted outline of a coatingroll having spaced orifices through which jets of air can be passed; and

FIG. 4 is a side elevation of the coating apparatus showing the web,pan, coating roll and associated elements.

Referring to the drawings, and more particularly to FIG. 1, coating roll1 has spaced ribs 2, grooves 3 and trunnions 4 which are to bejournalled in suitable bearing blocks (not shown).

In FIG. 2, the web-supporting surface of the roll has a plurality ofuniformly spaced cylindrical knobs 5.

In FIG. 3, the jacket of the coating roll has a plurality of uniformlyspaced orifices 6 which communicate with the hollow central part of theroll. If desired, orifices can be provided in the grooves or spaces ofthe rolls shown in FIGS. 1 and 2.

In FIG. 4, at least one of guide rollers 6 and 7 may be provided withsurfaces like the coating rollers of FIGS. 13. In this figure an airknife 8 is provided adjacent the coating pan 9. It can be connectedthrough any suitable pipe to a source of compressed air. The Web Wpasses around guide roller 6 then down under coating roll 1 where itcontacts with the coating liquid in the coating pan, then past the airknife where a jet of air further smooths the coating. The coated webthen passes around guide roller 7 and thence to a drying chamber,wind-up device or to a further web treating zone or apparatus.

Suitable air knives are disclosed in US. Patents 1,590,- 417, 1,980,923,2,139,628, 2,252,345 and 2,423,768.

The coating roll may vary considerably in length and diameter and thewidth depends upon the width of the web to be coated. Usually the rollwill be of small diameter from about cm. to 15 cm.

When the coating roll is heated prior to coating, the roll acquiresapproximately the temperature of the hot air current, since it is notcooled by the wet film.

The apparatus of this invention is useful in coating dispersions ofmagnetic particles in colloid binding agents, in coating diazo andbichromated colloid dispersions, in coating baryta layers on paper websduring the manufacture of photographic papers, and for coating otherfilmforming liquids. Suitable film supports include cellulose esters,e.g., cellulose triacetate, cellulose acetate/butyrate; superpolymers,e.g., polyvinyl chloride (co) vinyl acetate; polyvinyl acetals, e.g.,formals, acetals; polystyrene; polyamides, e.g., polyhexamethyleneadipamide, and polyesters e.g., polyethylene terephthalate, polyethyleneterephthalate/isophthalate; esters formed by condensing terephthalicacid and dimethyl terephthalate with propylene glycol, diethyleneglycol, tetramethylene glycol or cyclohexane 1,4 dimethanol(hexahydro-p-xylene alcohol). The vinylidene chloride copolymer-coatedoriented polyester films of Alles US. 2,779,684 can be coated inaccordance with this invention.

The invention is not limited to the coating of gelatinsilver halideemulsions. It can be applied in coating other simple and mixed aqueoussilver halide emulsions. The emulsions can contain binding agents otherthan gelatin or mixtures of gelatin and such binding agents. Suitablebinding agents include polyvinyl alcohol and acetals thereof, polyvinylpyrrolidone, polyvinyl lactams, cellulose esters, dextrin and dextran,the latter two, as well as polyvinyl pyrrolidone and polyvinyl lactamsbeing mixed with gelatin.

The use of the guiding rolls with a small area of contact with the weband the provision of an insulating cushion of air has the advantage thatit keeps a gelatin layer from melting. It often happens that emulsionsmelt in the first few meters after startup of coating with conventionalsmooth coating rolls, so that the material in that section of the webbecomes unfit for use. Melting of the emulsion can be prevented by usingas guide or conveyor rolls those made according to this invention which,by reason of their special surface construction prevent any substantialheat transfer from roll to emulsion. It has been demonstrated thatmelting of the emulsion is reliably prevented when rolls of thespecified kind are employed.

The process and apparatus of this invention have the important advantagethat they permit improved operation of the coating process and providecoated webs with layers of uniform thickness.

I claim:

1. A process for applying a thin coating of an aqueousgelatin-containing silver halide emulsion to the surface of an organicpolymer film support characterized in that during the applying of theaqueous emulsion there is provided a roll having a plurality ofuniformly spaced orifices communicating with a hollow interior throughwhich a gas is forced outwardly to form an insulating gaseous layer oflow thermal conductivity adjacent the back surface of the film supportto prevent heat transfer between the roll surface and the coating.

2. A process according to claim 1 wherein the surface of the roll ismade of organic plastic material of low thermal conductivity and the gasis air.

References Cited UNITED STATES PATENTS 1,699,349 1/1929 Dailey 117-342,040,529 5/1936 Pearl 118-419 2,176,835 10/1939 Cumfer 117-1152,269,169 1/1942 Van Derhoef et al. 117-34 2,542,819 2/1951 Kropa118-428 2,995,469 8/1961 Le Claire 117-34 3,065,098 11/1962 Brooks117-34 MURRAY KATZ, Primary Examiner.

1. A PROCESS FOR APPLYING A THIN COATING OF AN AQUEOUSGELATIN-CONTAINING SILVER HALIDE EMULSION TO THE SURFACE OF AN ORGANICPOLYMER FILM SUPPORT CHARACTERIZED IN THAT DURING THE APPLYING OF THEAQUEOUS EMULSION THERE IS PROVIDED A ROLL HAVING A PLURALITY OFUNIFORMLY SPACED ORIFICES COMMUNICATING WITH A HOLLOW INTERIOR THROUGHWHICH A GAS IS FORCED OUTWARDLY TO FORM AN INSULATING GASEOUS LAYER OFLOW THERMAL CONDUCTIVITY ADJACENT THE BACK SURFACE OF THE FILM SUPPORTTO PREVENT HEAT TRANSFER BETWEEN THE ROLL SURFACE AND THE COATING.